1. Field of the Invention
The present invention relates to a surface acoustic wave filter for use as, e.g., a band filter and, more particularly, to a transversal type surface acoustic wave filter in which an electrode including an electrode layer of Al or an Al alloy is provided on a crystal substrate.
2. Description of the Related Art
Known transversal type surface acoustic wave filters are used as IF filters in portable telephones. However, the transversal type surface acoustic wave filters have problems in that the insertion loss is large.
Thus, heretofore, different types of surface acoustic wave filters have been proposed which include unidirectional electrodes for reducing the insertion loss.
IEICE Trans. '86/10 Vol. J69-C No. 10 (p 1297–1308) (Non-patent Document 1) describes a method of designing a SPUDT (Single Phase Uni-Directional Transducer) which is a kind of unidirectional electrode. According to the description of Non-patent Document 1, when a SPUDT is designed, the reflection is weighted such that the reflected wave in IDT is cancelled out. Thereby, the loss is reduced, and moreover, a ripple, caused by triple transit echo (TTE), is reduced.
On the other hand, the bandwidth of a filter is determined by the weighted excitation in the IDT. Accordingly, to improve the shape factor, i.e., the steepness of the filter characteristic and realize a narrow bandwidth, the number of pairs of electrode fingers must be increased in the IDT. Therefore, there have been problems in that it is difficult to reduce the size of the filter.
To solve the above-described problems, U.S. Pat. No. 5,703,427 (Patent Document 1) discloses a configuration of a SPUDT with which the size is reduced, the shape factor is improved, and a narrow band is produced. That is, reflection elements are arranged in the forward and reverse directions in the IDT, such that local resonance is generated, and thereby, the duration time of an impulse is increased. Thus, the size of the IDT is reduced.
Moreover, Japanese Unexamined Patent Application Publication Nos. 2000-91869 (Patent Document 2), 2001-53581 (Patent Document 3), and 2001-332955 (Patent Document 4) disclose surface acoustic wave filters using SPUDT electrodes. Moreover, Patent Documents 2 to 4 describe structures in which SPUDT electrodes made of Al are provided on Y-cut crystal substrates. Furthermore, Patent Documents 2 to 4 describe that preferably, the electrode film thickness ratio h/λ is in the range of 0.005 to 0.035, wherein h represents the electrode film-thickness, and λ represents the wavelength of a surface acoustic wave.
According to the structure using a SPUDT in the surface acoustic wave filter described in Patent Document 1, the size is significantly reduced, and the shape factor is improved, as described above. Although significant reduction in size has been achieved, requirements for size-reduction and the improvement of the shape factor or the formation of a narrow band are contradictory to each other. Thus, the known technique described in Patent Document 1 does not sufficiently meet the above-described requirements.
Particularly, reception-side band filters for use in CDMA One of portable telephones are required to have excellent characteristics such as a narrow band e.g., having a specific bandwidth of about 1.1%, and a superior shape factor. Moreover, surface acoustic have filters having a reduced size have been in great demand.
Thus, the surface acoustic wave filter described in Patent Document 1 is insufficient to meet the above-described severe requirements. Particularly, the size reduction of elements is required. According to the surface acoustic wave filter using an R-SPUDT described in Patent Document 1, it is very difficult to further reduce the size of the surface acoustic wave filter while ensuring the filter has a narrow band and a superior shape factor.